1. Field of the Invention
The present invention relates to a light-emitting device, an electronic device, and a lighting device each using organic electroluminescence (EL).
2. Description of the Related Art
A light-emitting element (also referred to as an organic EL element) using organic EL has been actively researched and developed. In the fundamental structure of an organic EL element, a layer including a light-emitting organic compound is provided between a pair of electrodes. By voltage application to this element, light emission from the light-emitting organic compound can be obtained.
An organic EL element, which has characteristics such as feasibility of being thinner and lighter, high speed response to input signals, and capability of direct current low voltage driving, has been expected to be applied to next-generation flat panel displays or lighting devices. A display device in which organic EL elements are arranged in matrix is considered to have advantages of a wide viewing angle and excellent visibility over a conventional liquid crystal display device. Organic EL elements can be formed in the form of a film and therefore can easily constitute large-area elements, and also have great potential as planar light sources applicable to lighting and the like.
A problem of organic EL elements is that its light-emitting properties (e.g., luminance) and reliability are impaired by heat.
Since organic EL elements are self-luminous elements, they generate heat to raise their temperature when driven. The luminance of an organic EL element is temperature-dependent and therefore a rise in the temperature of an organic EL element might diminish its luminance. Further, a rise in the temperature of an organic EL element might degrade organic compounds forming the element, which reduces the lifetime of the element.
In view of the foregoing, research and development are being conducted on techniques for reducing a rise in the temperature of an organic EL element by emission of heat generated by the element to the outside thereof.
For example, Patent Document 1 discloses an organic EL panel in which a heat dissipating layer is provided on a light-emitting element.
Another problem of organic EL elements is that their reliability is impaired by impurities such as moisture or oxygen entering from the outside.
When impurities such as moisture or oxygen enter an organic compound or a metal material included in an organic EL element from the outside thereof, the lifetime of the organic EL element might be significantly shortened. This is because an organic compound or a metal material included in the organic EL element reacts with the impurities such as moisture or oxygen and is consequently degraded.
In view of the foregoing, research and development are being conducted on techniques for sealing an organic EL element to prevent entry of impurities.
For example, known sealing techniques are the technique by which an organic EL element is sealed between a support substrate provided with the element and a thin film (hereinafter, also referred to as film sealing) and the technique by which a support substrate and a sealing substrate are bonded to each other with resin, glass frit, or the like and an organic EL element is sealed between the support substrate provided with the element and the sealing substrate.
In the film sealing, productivity is low and accordingly problems such as high cost arise. In contrast, cost is low and productivity is high in the technique for sealing an organic EL element with a pair of substrates using resin, glass frit, or the like, and hence it is considered a preferable sealing technique.
For example, in the organic EL panel disclosed in Patent Document 1, a substrate provided with the light-emitting element and the like and a sealing substrate to which a drying agent is attached are bonded to each other with an ultraviolet curable resin.
As the sealing substrate, a metal substrate using a metal material or an alloy material is known.
Also known are so-called hollow sealing by which a space where an organic EL element is sealed is filled with gas, solid sealing by which such a space is filled with solid, and liquid sealing by which such a space is filled with liquid.
<Light-Emitting Device Employing Liquid Sealing>
A problem is that, since complete removal of impurities in the liquid is difficult, impurities such as moisture and oxygen remaining in the liquid adversely affect an organic EL element.
<Light-Emitting Device Employing Solid Sealing>
When a light-emitting device is subjected to impact such as external force through the sealing substrate side, an organic EL element in contact with the sealing substrate with the solid interposed therebetween tends to be affected by the external force. This makes the element likely to have a short circuit between its anode and cathode or to be broken. This is because solid is more difficult to deform than liquid or gas and cannot easily absorb or reduce impact such as external force. Such a problem also lies in the adoption of the above-described film sealing.
Furthermore, stress might concentrate at an extraneous substance derived from the solid which has entered the light-emitting device during process, which allows the extraneous substance pressed onto the element to cause a short circuit between the anode and cathode of the element.
<Light-Emitting Device Employing Hollow Sealing>
In a light-emitting device employing hollow sealing, owing to a space containing gas between a sealing substrate and an organic EL element, it is possible to prevent direct application of external force to the organic EL element, which might break the element, even when impact such as external force is applied through the sealing substrate side. Therefore the yield and reliability of the light-emitting device employing hollow sealing are higher than those of a light-emitting device employing solid sealing or liquid sealing.